The present invention relates to a method for electrolytic coloring of aluminum or aluminum alloys (hereinafter the term "aluminum" is used to indicate both aluminum and aluminum alloys). More particularly, it is concerned with a method for electrolytic coloring of aluminum which comprises preliminarily treating anodized aluminum by applying a voltage of positive voltage wave form followed by electrolytic coloring by applying a voltage of asymmetrical alternating current, whereby uniform and good coloring can be efficiently applied onto the aluminum surface.
In electrolytic coloring of aluminum, various methods have been proposed to improve the throwing power (or coloring ability) and coloring speed. For example, for improvement of the throwing power in coloring, the following have been proposed: a method in which a newly developed electrolyte is used (Japanese Patent Publication No. 11119/1985), a method in which in electrolytic coloring, the voltage is increased or applied in a newly developed manner (Japanese Patent Publication No. 23663/1979 (corresponding to U.S Pat. No. 4,070,255), Japanese Patent Publication No. 46557/1983, Japanese Patent Application Laid-Open No. 145798/1984, Japanese Patent Publication Nos. 34287/1974, 49408/1977, 27953/1982 and 4503/1978, a method in which prior to electrolytic coloring, specified direct current anodization is applied (Japanese Patent Publication Nos. 13859/1979, 25898/1979 (corresponding to U.S. Pat. No. 4,021,315), 23664/1979, 52037/1983 (corresponding to U.S. Pat. No. 4,316,780), and 39237/1983), and so forth. In addition, to increase the coloring speed in electrolytic coloring, a method of modifying an electrolyte (Japanese Patent Publication Nos. 11119/1985 and 23663/1979), a method of using special material for counter electrode (Japanese Patent Publication No. 13440/1985), and so forth have been proposed.
When practicing these known electrolytic coloring methods and particularly the alternating current electrolytic coloring method, either the throwing power or the coloring speed is improved but the other is improved only insufficiently or rather tends to be reduced.
In particular, the method of coloring by controlling the alternating current with a resistor or thyrister as disclosed in Japanese Patent Publication Nos. 4503/1978, 34287/1974 and 27953/1982 has a problem in that the coloring effect cannot be sufficiently obtained because the barrier layer is not adjusted. The method in which the barrier layer is previously adjusted and then electrolytic coloring is applied using negative direct current with a positive pulse voltage applied, as disclosed in Japanese Patent Publication No. 52037/1983 (corresponding to U.S. Pat. No. 4,316,780) has problems in that controlling the current becomes markedly complicated and equipment cost is increased, both of which are disadvantageous from an economic standpoint.